Preheat oven to 400 degrees.,Chill 4 individual souffle dishes, each with 1 1/2-cup capacity.,Melt 1 tablespoon of butter in saucepan and add onion.,Cook, stirring, until wilted.,Add tuna and cook, stirring, until warm.,Put mixture on a board and chop until coarse-fine.,Meanwhile, bring milk barely to simmer and remove from heat.,Remove souffle dishes and brush inside of each with butter, using total of 2 tablespoons.,Sprinkle inside of each dish with tablespoon of grated cheese.,______,Melt 3 tablespoons of butter and add flour, stirring rapidly with wire whisk.,When blended and smooth, add milk, stirring rapidly with whisk.,Stir until thickened and smooth.,Blend cornstarch and water and stir into simmering mixture.,Add mustard, Worcestershire sauce, Tabasco sauce, salt and pepper.,Add egg yolks while beating briskly with whisk.,Cook 5 seconds, no longer, and remove from heat, beating.,Scrape mixture into mixing bowl.,Stir in tuna mixture.,Set aside briefly to cool slightly.,Beat egg whites until stiff.,Fold cheese cubes into sauce mixture.,Add about 1/3 of beaten whites and beat in rapidly until well distributed.,Add remaining whites to mixture and fold in with plastic spatula.,Fill souffle dishes with mixture and smooth tops.,Run thumb at a depth of about 1/2 inch neatly around inside of each dish to clean the top.,Arrange dishes on baking sheet and place in oven.,Bake 20 minutes until well puffed and golden brown on top.,Serve immediately.